Volltext: Helmholtz's treatise on physiological optics. Volume 2. Edited by James P. C. Southall. Translated from the 3rd German edition (2)

372 
The Sensations of Vision 
[314. N. 
foveal vision to be the luminosity of a magnesium oxide surface illu¬ 
minated by 0.033 metre-candle. At this intensity the curve, as shown 
in Fig. 70, begins to bend into the steeper branch, and here for the first 
time the daylight mechanism is appreciably involved, until when the 
intensity gets to be about ten times as great, it is the sole factor in the 
power of the retina to make space-discriminations. Thus it may be said 
that at the border between twilight vision and daylight vision there is 
some sudden change in the relations between intensity of illuminations 
and visual acuity, the daylight mechanism being dependent on the in¬ 
tensity of illumination in a much greater proportion. 
Likewise there are marked differences between the photopic 
mechanism and the scotopic mechanism with respect to the duration 
and course of the process of stimulation. It is true we are not yet in a 
position to describe exhaustively the course of the stimulation in the 
two cases. However, it may be stated that the sensational response to 
a short-lived stimulus is more sluggish with the scotopic mechanism 
than with the photopic mechanism. This is shown most distinctly by 
rapidly intermittent stimulation ; for example, by interposing a rotating 
sectored disc between the source of light and the eye, or simpler still, 
a rotating disc with alternating black and white sectors. As the speed 
of rotation, slow to begin with, is gradually increased, the sectors, 
which at first can clearly be distinguished, become more and more 
indistinct, the alternation from bright to dark being perceived merely 
as so-called “flicker”, until finally at a certain speed the disc appears 
a uniformly steady grey. Knowing the angular speed and the number 
of sectors, the number of interruptions can be calculated at which 
flicker just ceases. It is true it is not altogether easy to determine this 
limit, because the direction of fixation must be kept constant, and the 
observation has to be made with a visual angle that it is not too large, 
say, from 3° to 5°: For the latter purpose, the greater part of the 
rotating disc can be covered with an opaque surface with an aperture 
of the correct size.1 
1 ^[For recent accounts of flicker the following may be consulted: 
H. Bender, Untersuchungen am LuMMER-PRiNGSHEiMschen Spectralflickerphoto- 
meter. Ann. d. Physik. XLV. 1914. 105-132.—W. W. Coblentz and W. B. Emerson, The 
relative sensibility of the average eye to light of different colors, and some practical ap¬ 
plications to radiation problems. Bull. Bur. Stand. Sei. paper No. 303. 1917. 167-236.— 
E. C. Crittenden and F. K. Richtmyer, An average eye for heterochromatic photometry 
and a comparison of a flicker and equality of brightness photometer. Bull. Bur. Stand.XIV. 
1918. 87-114. — U. Ebbecke, Über das Augenblicksehen II. Über das Sehen im Flimmer- 
licht. Pflügers Arch., 1920. CLXXXV. 181-195.—■ Idem, Über das Sehen im Flim¬ 
merlicht, Pflügers Arch CLXXXV. 1921. 196-223. — C. E. Ferree and G. Rand, 
Flicker photometry. I. The theory of flicker photometry. II. Comparative studies of 
equality of brightness and flicker photometry with special reference to the lag of visual 
sensation. Trans. Ilium. Engin. Soc. 1922. 50 pages. — H. E. Ives, Studies in the
	        
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